In a conventional optical disc system, to sense the position of the laser beam in relation to the track on the disc, the main laser beam creates a reflection from the disc. The reflection is typically picked up by 4 photo-diode sensors (or a photo detector array). FIG. 1 is a conceptual diagram illustrating how such a photo-diode configuration is laid out in relation to the track direction. The outputs of the 4 photo-diodes (when the laser beam is focused on the disc) are shown as signals A, B, C and D, respectively.
The tracks on an optical disc (or disc) form a long spiral. As the disc spins, a laser follows the spiral from the inner diameter (ID) to the outer diameter (OD) of the disc. However, for discs with run out, the center of the spiral is not the center of rotation of the spiral. As the disc rotates, the tracks move radially relative to a fixed point (e.g., laser spot). To reduce such an effect of additional errors created by placing the laser spot on the tracks, a closed loop control system is used. While the closed loop control is operating, the lens moves radially with the disc so that the laser spot is held on the center of the track. The radial motion between the lens and the disc is defined as the run out.
The track position (i.e., the location of the laser spot relative to a center of the track) is detected by imaging the laser on a photo detector array. Diffraction causes a slight change in intensity on the two different sides of the photo detector array when the relative position of the laser spot and the track center changes. The difference in intensity on the two different sides of the photo detector array is called a push-pull signal. The push-pull signal is proportional to the tracking error signal.
If the laser does not shine directly through the center of the lens, an image is moved to one side and the image shows up in the push-pull signal change. Such an effect is defined as the center error. The center error cannot be distinguished from the push-pull effect by examining one laser spot alone. To obtain an accurate track position, a second measurement is taken on one-half track away from where the first measurement was taken. With the second measurement, the center error is common to the first measurement, but the push-pull effect is reversed. By combining the first and the second measurements, an accurate track position can be determined. For DVD ROMs, a phase detection method is used to detect the track position. The phase detection method is mostly immune to the effect of center error.
Referring to FIG. 2, a typical center error waveform in the presence of a disc run out during tracking mode is shown. Run out is proportional to center error. As the disc moves radially, with a period of one spindle revolution, the laser follows the center of the tracks. The radial motion of the lens induces a center error that is synchronous to the rotation of the disc.